Test method and system for variable gain amplifiers



Sept i8, 1945 J. w. BAYLEss 2,385,186

TEST METHOD AND SYSTEM FOR VARIABLE GAIN AMPLIFIRS y Filed June 26, 1942Juris' 1%' 4x/55.5

IN VENTO R.

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I Patented Sept. 18, 1945 James W. Bayless, Los Angeles,

Calif., assignor to Radio Corporation of America. a corporation oi'Delaware Application June ze, im, serial No. 443.516

iz calm. (ci. irs-175.31)

This invention relates to electrical current transmission systems andparticularly to a method of and means for testing variable gainportionsof such transmission systems.

In electrical transmission systems employed Afor sound recording,broadcasting. public address systems and the like, it has been founddesirable to vary the amplification or attenuation ofthe currents beingtransmitted. For instance, in sound recording wherein a photographicrecord is made on a motion picture film, the physical limitation of thesound track portion of the nlm necessitates the reduction of the rangeof the original signals in order to accommodate them Within the soundtrack area. 'Ihe manner of accomplishing the variation in gain orattenuation is generally by the use of electronic vacuum tubes such asthe variable mu type, these tubes usually being used in a push-pullrelationship to avoid introducing distortion into the signal by thevariations in control voltage on the grids of the tubes. 'Ihe greaterthe imbalance of these tubes and circuits, the greater the distortionintroduced.

Thus, it is particularly desirable to have not only the circuitsbalanced, but also to use balanced or identical tubes in each half ofthe pushlpull system. The physical elements ol the circuit such as theresistances, capacitances and inductances can be easily measured, butthis is not true of the vacuum tubes which are usually not identicalover their entire operating range.

Although these tubes may be balanced statically at one or more gridbiases, it is not an indication that they are in balance dynamically.Thus, to select tubes which are dynamically balanced, it is necessary toprovide a test system wherein any discrepancies acteristics of the tubesmay be quickly realized.

Means'of balancing push-pull tubes and circuits at one or more settingsof grid bias are well known in the art', such as shown in Lavoie PatentNo. 1,948,303 of February 20, 1934 and Meisner Patent No. 1,946,092 ofFebruary 6, i934.

prior art systems, however, none provide a truly dynamic test for eachand every point on the characteristic curve. The prior art systems areconcerned with tubes operating with a fixed bias while applicantsproblem involves the balancing of push-pull tubes having a varying gridbias such as found in compressor and expander systems.

The principal object of the invention, therebetween the actual operatingcharmung the balancing Y of filename fgain 'amplifier -y .electronicjdevices;

.Another/object Aoigtliinvention is to provide limprovedmeansforfcilitating the testing 'of a variable gainfamplifler' tofacilitate the selectionbofbalanced vacuum tubes therefor.

A ,further` object ,of the invention is to provide a test `system'forV avariable gain amplifier unit, wherein a rapid dynamic balance test maybemade., j

A further object of the invention is to provide a dynamic test systemfor. variable gain amplifiers to facilitate the selectionof Vbalanced oridentical vacuumtubes.,

Although the novel,'f'eaturesV which 'arebes lieved 'to be,characteristic of thexinvention :are

in thefclaims appointed out with particularity v y pended herewith, themanner of its-organization and the mode of its operation will bebetterun-` derstood by referring to the following description, read inconjunction with the accompanying drawing' in which the single drawingis a schematic circuit of a compressor unit embodying the invention.l

` Referring to the drawing, the variablev gain 4 amplifier shown thereinis of 'the type disclosed Y former 20 of an amplifier 2i.

balanced, the variations in rectified current from put circuits of whichare capacitance network I2 to a fixed gain push-pull roi' the fixed gaintubes II and Ii is and claimed in Singer Patent No. 2,255,683 ofSeptember 9, 1941, wherein the gain of a pair of variable mu tubes i and6, is controlled by a portion of the rectified output thereof.` Brieflydescribing the operation of the compressor, a signal is impressed overan input transformer 8 to the grids of variable mu tubes 5 and 6. theoutcoupled by a resistance- The output transmitted over a transformer ilto a sound recorder or other transmission elements of the system, aportion of the output being impressed over conductors il, when a switchIl is closed, on. an input trans- The output of amplifier 2l istransmitted over transformer 22 to a rectifier 2l. The rectifiedcurrents from rectifier 23 are fed over conductor 25 to a. resistance 10, which varies the biasing potential on the grids of tubes S and 6 andthereby varies their gain as disclosed in detail in the above-mentionedSinger patent. l

` It is realized from the above that should either tubes 5 or 6 or theirrespective circuits be unamplier including tubes ill and Il.

the rectifier 23 will not be balanced out or neutralized and will beintroduced as longitudinal currents into the push-pull system and appearin the signal. Since it is desirable that the compressor operate asrapidiy as possible, the amount of filtering introduced is limited,which increases the need for good balance to eliminate harmonics.

From the above it is realized that a very ac- 4curate balance isrequired not only in the circuit, but also in the tubes which form aportion of the circuit. It has been found that the static testing oftubes for these circuits is unsatisfactory as balanced tubes from astatic test have been sufficiently unbalanced from a dynamic standpointto be unsuitable for service as variable gain amplifiers wherein theirgain is varied from time to time. For fixed gain tubes such as tubes land Il the static test may be satisfactory, although the present systemis also suitable for obtaining a dynamic balance in the tubes I0 'andIl.

The above circuitshows elements which may be combined to provide twotypes of tests, one test involving the impression of a varying amplitudeconstant frequency current of 9,000 cycles over transformer 8 tosimulate the application of a signal through the system. Thus, a 9,000cycle oscillator 30 is provided with a potentiometer 3| for varying theamplitude of the output of the oscillator and a double-throw switch 32is employed to connect the oscillator 30 to the transformer 8 when theswitch 32 is in its left-hand to aid in balancing the circuit and thetubes. For extreme unbalanced conditions, however, it is necessary tosubstitute tubes with more identical dynamic characteristics.

The other test for balance is accomplished by applying the 9,000 cyclevariable amplitude frequency directly on the primary of transformer bythe use of an oscillator 41 having a variable potentiometer 43 in itsoutput circuit. The 9,000 cycle current from oscillator 41 is appliedwhen switch 50 is closed and when so used the switch 32 is thrown to itsright-hand position to disconnect the oscillator 30 and to terminate thetransformer 8 across resistance 34. The 60 cycle current is stillimpressed across the resistance 26 and theA switch IT' is now opened.

position. When the oscillator 30 is not used, the

switch 32 may be thrown to its right-hand position and a terminatingimpedance 34 is then connected across the primary of the transformer 8.A sixty cycle constant amplitude oscillator 36 is used in both tests.The oscillator is`connected across the grid biasing resistance 28 over aload resistance 31 when a switch 33 is closed. This 60 cycle oscillatorsupplies the test current. A

4,000 cycle low pass lter is also used in both 40 tests. The output ofthe filter 40 isvamplifled in a high gain amplier 4| and is applied to ameter 42. The amplifier 4| should have a high gain in the neighborhoodof 90 db in order to raise the amplitude of unbalanced componentsto avalue where they will be readable on the meter 42.

To apply the rst test, the switch 32 is thrown to its left-handpositionand the switches i1 and 38 are closed. The current from 9,000 cyclesfrom oscillator 30 should now be manually varied in amplitude by meansof the potentiometer 3|, this current passing through the variable gaintubes 5 and 6, fixed gain tubes Il and Il, through amplier 2l, andthrough the rectier 23. The variations in amplitude oi the 9,000

cycle current will appear as direct current voltage variations acrossthe resistance 23, and will thereby simultaneously vary the bias andgain of tubes 5 and 8 as this voltage is impressed ir` an in-phaserelationship. If any unbalance exists at certain amplitudes of the 9,000cycle cur..

rent controlling the gain of tubes 5 and 3, moreor-less of the 60 cyclecurrent will appear at meter 42, the 4,000 cycle low pass iiitereliminating the 9,000 cycle tone. In this manner, the grids of tubes 5and 6 are varied in gain over their complete operating range of gridbias, while transmitting currents corresponding to a normal signal. Ithas been found in using such a dynamic test that two tubes may beperfectly balanced statically at the extremities oi their grid biasrange and be very much of! balance in their middle range. The above watwin disclose any such discrepancies. A variable tap 44 on 'Ihe above twotests differ solely in the point of application of the 9,000 cyclevariable amplitude current, the first-mentioned test being preferred asit more nearly simulates the normal operation of the compressor andtests the unbalance of the entire compressor unit.

I claim as my invention:

1. In a test system for a push-pull variable gain amplifier, thecombination of a pair of vacuum tubes in push-pull relationship, meansfor applying a relatively low cyclic voltage variation to the gaincontrol electrodes of said tubes in phase to simultaneously vary thevoltages on said electrodes in the same direction, means for applying-arelatively high frequency voltage variation to the control grids of saidtubes in phase` opposition to simultaneously vary the voltage on saidelectrodes in opposite directions-and means for measuring the amount ofsaid in-phase voltage introduced in the output circuit of said variablegain amplifier during said voltage variations.v

2. The combination in accordance with claim 1 in which means areprovided for varying the amplitude of said high frequency voltageimpressed on said tubes and impressing a direct current voltage on thecontrol electrodes 0f said tubes,

. said direct current voltage varying in amplitude in accordance withthe variations in amplitude of said high frequency voltage and varyingthe voltage on said electrodes in the same direction at any instant.

3. The combination in accordance with claim 1 in which' said lowfrequency voltage is a sixtycycle constant amplitude voltage, thefrequency of said high frequency voltage is approximately 9,000 cyclesand means are provided for rectifying said 9,000 cycle voltage.

4. The method of testing a push-pull variable gain amplier comprisingimpressing a relatively low frequency voltage on the control grids .ofthe tubes of said amplifier in an in-phase relationship tosimultaneouslyvary the voltage on said control grids in the same direction at anyinetant, simultaneously .impressing a relatively high frequency varyingamplitude voltage on said resistance 43 is provided 76 urine the amountof low frequency voltage present in the output circuit of said tubesduring said voltage variations.

5. The method of testing in accordance with claim 4 in which saidhighfrequency voltage is manually varied in amplitude over a predeterminedrange, said high frequency being rectified and applied to saidcontrolgrids to vary the voltage thereon in the same direction simultaneously.

6. The method of testing in accordance with claim 4 in which said highfrequency voltage is rectified after amplification by said tubes, saidrectified current Icontrolling the voltage im' pressed on said tubes tovary their grid potential in the same direction at any instant.

7. A test system for a push-pull variable gain amplifier comprising apair of vacuum tubes connected in a push-pull re1ationship, means in the`common grid circuit of said tubes to vary the grid bias thereofsimultaneously in the same direction, means for impressing aperiodically varying current on said means, means for generating a highfrequency voltage, means for impressing said voltage on the grids oi.'said tubes to vary the grid bias in opposite directions at any instant,means for rectifying said voltage after amplication by said tubes, meansfor impressing the products of rectification on said rst-mntioned means,and means for measuring the amplitude of said periodically varyingcurrent obtained in the output ,circuit of said tubes during said gridbias variations.

8. A test system for a push-pull amplifier comprising a pair of vacuumtubes connected in a push-pull relationship, means in the common gridcircuit of said tubes for varying the grid bias thereof, means forgenerating a low frequency current, means for generating a relativelyhigh frequency current for varying the voltage on said grids in oppositedirections at any instant, means for rectiiying said high frequencycurrent, means for impressing said low frequency current and saidrectiied current on said mst-mentioned means to simultaneously vary thevoltage on said grids in the same direction at any instant, and

means for measuring the amount of low frequency current in the outputcircuit of said'vacuum tubes during variations in said voltages on saidgrids.

9. A test system in accordance with claim 8 in which th'e amplitude ofsaid high frequency current is varied between predetermined limits.

10. A test system in accordance withclaim 8 inl in phase opposition tovary the voltage on said grids of said tubes in opposite directions atany instant, means for generating a relatively low frequency voltage,means for impressing said low frequency voltage on said tubes in phaseto vary` th'e voltage on said grids in the same direction at anyinstant, and means for simultaneously impressing a direct currentvoltage on said tubes to vary the voltage on said grids in the samedirection at any instant.

12. A test system in accordance with claim 11 in which said directcurrent voltage is varied in amplitude in accordance with thevariations-in amplitude of said high frequency voltage.

JAMES W. BAYLESB.

